A method and system for measuring vibration using an all-fiber optic cable based on frequency domain interferometry

Abstract

This invention discloses an all-fiber vibration measurement method and system based on frequency domain interferometry, belonging to the field of vibration measurement technology. The key technical points are: emitting a sequence of repeating pulsed lasers with broad spectral characteristics; emitting a square wave signal with the same repetition frequency as the pulsed laser, and adjusting the timing of the optical path operation according to the square wave signal; collecting the signal light and reference light formed after single pulses from the pulsed laser are used to measure the displacement of the vibrating object; amplifying the signal light and reference light when the optical path is open to obtain an optical signal with frequency domain interferometry characteristics; and performing a Fourier transform on the frequency domain interferometry characteristics of the optical signal to obtain the vibration displacement of the vibrating object. This invention enables accurate signal analysis using Fourier transform, achieving vibration process measurement at the kHz level.

Description

Technical Field

[0001] This invention relates to the field of vibration measurement technology, and more specifically, to an all-fiber vibration measurement method and system based on frequency domain interferometry. Background Technology

[0002] Vibration is the most common form of motion that occurs when an object is subjected to external forces. By measuring the vibration process, the response behavior of an object to external forces can be accurately analyzed. Therefore, vibration measurement technology has wide applications in industry and science and technology. In recent years, non-contact and interference-resistant optical vibration measurement technology has become a research hotspot, and various optical vibration measurement systems based on laser triangulation, light intensity method, holography, interferometry, and speckle method have emerged.

[0003] Currently, existing optical frequency domain reflection technology uses continuous wave modulation for length measurement. It typically employs a tunable laser with linearly adjustable optical frequency as the light source. The length of the fiber under test is demodulated using the beat frequency signal generated by the superposition of two beams that have experienced different transmission times. An oscilloscope or RF spectrum analyzer is usually used to analyze the signal. This technology is used in distributed fiber optic sensing with lengths in the kilometer range, achieving millimeter-level accuracy, and is mainly used for fiber optic fault location, as illustrated in Chinese Patent Publication No. CN101650197A. In addition, existing frequency domain interferometry technology uses the spectral coherence principle of broadband light for length measurement. It typically uses a spontaneous emission light source or halogen lamp with a spectral width of tens of nanometers as the system light source. The phase information of the spectral interference signal generated by the superposition of two beams that have experienced different transmission times is used to demodulate the length information of the optical path under test. An optical spectrometer is usually used to analyze the signal. This technology is used in millimeter-range distance measurement, achieving micrometer-level accuracy, as illustrated in Chinese Patent Publication No. CN205120239U. Additionally, a Chinese patent with publication number CN104197844B was found, disclosing a method and apparatus for measuring absolute distance using all-fiber frequency domain interferometry. This method employs a continuously emitted spontaneous emission light source, with a fiber optic spectrometer at a sampling rate of Hz for signal recording. However, during dynamic displacement measurements such as vibrations, the rapid movement of the target will cause ambiguity in the signal within the sampling time of a single signal. That is, multiple wavelengths in a single signal correspond to multiple positions of frequency domain interference phase. Therefore, this method can only perform static distance measurements.

[0004] Therefore, how to research and design an all-fiber vibration measurement method and system based on frequency domain interferometry that can overcome the above-mentioned defects is a problem that we urgently need to solve. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method and system for measuring vibration using an all-fiber optic system based on frequency domain interferometry. By using a broadband pulsed laser and a high-speed fiber optic spectrometer to improve the sampling rate of the system, and by using a signal generator to precisely control the measurement timing, the system achieves high-speed measurement signal timing control and high-speed sampling. At the same time, the system also employs a fiber optic acousto-optic modulator and a fiber optic amplifier to suppress dynamic ambiguity and amplify the signal, ultimately achieving kHz-level vibration process measurement.

[0006] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0007] Firstly, a method for measuring vibration using an all-fiber optic cable based on frequency domain interferometry is provided, comprising the following steps:

[0008] It emits a series of repeating pulsed lasers with broad spectral characteristics;

[0009] It emits a square wave signal with the same repetition frequency as the pulsed laser, and adjusts the timing of the optical path operation according to the square wave signal;

[0010] The signal light and reference light generated after single pulses from pulsed lasers are used to measure the displacement of a vibrating object under test are collected.

[0011] When the optical path is open, the signal light and the reference light are amplified to obtain an optical signal with frequency domain interference characteristics.

[0012] The vibration displacement of the vibrating object is obtained by performing a Fourier transform on the frequency domain interference characteristics of the optical signal.

[0013] Furthermore, the pulsed laser has a spectral half-width of not less than 5 nm, a pulse half-width of not more than 20 ns, and a repetition frequency of not more than 50 kHz.

[0014] Furthermore, the frequency domain interference characteristic expression of the optical signal is specifically as follows:

[0015]

[0016] Where I(f,t) represents the optical energy spectral density, f is the optical frequency, and t is the pulse laser emission time; E0(f) represents the optical electric field intensity; and T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the pulse laser emission time. This represents the inherent phase difference between the signal light and the reference light.

[0017] Furthermore, the specific formula for calculating the vibration displacement of the vibrating object under test is as follows:

[0018] T(t) = 2d(t) / c

[0019] Where T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the moment of pulse laser emission; d(t) represents the vibration displacement of the surface of the vibrating object due to vibration; and c represents the speed of light in a vacuum.

[0020] Secondly, a frequency-domain interferometry-based all-fiber vibration measurement system is provided. This system is used to implement the frequency-domain interferometry-based all-fiber vibration measurement method described in any one of the first aspects, including:

[0021] A broadband pulsed laser is used to emit a series of repeating pulses of laser light with a broadband spectrum.

[0022] The optical path control circuit is used to emit a square wave signal with the same repetition frequency as the pulsed laser, and to regulate the timing of the optical path operation according to the square wave signal.

[0023] The displacement measurement circuit is used to collect the signal light and reference light generated after the single pulse in the pulsed laser measures the displacement of the vibrating object;

[0024] The signal processing circuit is used to amplify the signal light and the reference light when the optical path is open, so as to obtain an optical signal with frequency domain interference characteristics.

[0025] A fiber optic spectrometer is used to perform Fourier transform on the frequency domain interference characteristics of optical signals to obtain the vibration displacement of the vibrating object under test.

[0026] Furthermore, the displacement measurement circuit includes an optical fiber circulator and an optical fiber probe;

[0027] The three ports of the fiber optic circulator are respectively connected to the output of the broadband pulsed laser, the input of the fiber optic probe, and the input of the fiber optic acousto-optic modulator in the signal processing circuit.

[0028] Furthermore, the fiber end face of the fiber probe is either uncoated or coated with a dielectric antireflection film to adjust the return loss, thereby controlling the intensity ratio of the reference light and the signal light.

[0029] Furthermore, the optical path control circuit is built into the broadband pulsed laser, and synchronously outputs a square wave signal with the same repetition frequency as the pulsed laser.

[0030] The trigger output terminal of the broadband pulsed laser is connected to the trigger terminal of the fiber optic acousto-optic modulator in the signal processing circuit, and the opening and closing of the fiber optic acousto-optic modulator is controlled according to the high voltage and low voltage of the square wave signal.

[0031] The trigger output terminal of the broadband pulsed laser is simultaneously connected to the trigger input terminal of the fiber optic spectrometer to control the synchronous operation of the fiber optic spectrometer.

[0032] Furthermore, the signal processing circuit includes an optical fiber acousto-optic modulator and an optical fiber amplifier connected sequentially along the signal output direction.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The present invention proposes an all-fiber vibration measurement method based on frequency domain interferometry, which uses a sequence of repeating pulsed lasers with broadband characteristics to measure vibration, thereby increasing the sampling frequency. It also precisely controls the measurement timing, suppresses dynamic ambiguity and amplifies the signal, and can use Fourier transform for accurate signal analysis, thus realizing the measurement of vibration process at the kHz level.

[0035] 2. The measurement system in this invention uses a broadband pulsed laser with a repetition frequency of up to 50 kHz and a fiber optic spectrometer with a matching repetition frequency, which can measure vibration processes with frequencies not exceeding 25 kHz.

[0036] 3. This invention uses the frequency domain interferometry principle to measure vibration displacement, which has high measurement accuracy and range. The upper limit of amplitude measurement can reach 6mm, the accuracy can be better than 0.01mm, and the resolution can reach 2.4μm.

[0037] 4. When this invention uses a pulsed laser with a half-width of no more than 20 ns as the system light source for measurement, the maximum vibration displacement is less than 2.4 μm within the pulse time, which is less than the measurement resolution, so it will not cause dynamic blurring. Attached Figure Description

[0038] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0039] Figure 1 This is a flowchart from an embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram illustrating the working principle of an embodiment of the present invention;

[0041] Figure 3 This is a measurement result diagram when the vibration frequency is 500Hz in an embodiment of the present invention. a is a schematic diagram of the vibration signal, and b is a schematic diagram of the power spectrum corresponding to the vibration signal.

[0042] Figure 4 This is a measurement result diagram when the vibration frequency is 2000Hz in an embodiment of the present invention. a is a schematic diagram of the vibration signal, and b is a schematic diagram of the power spectrum corresponding to the vibration signal.

[0043] Figure 5 The graph shows the results of the test in this embodiment of the invention, where a represents the positive direction and b represents the negative direction.

[0044] The attached diagram shows the markings and corresponding component names:

[0045] 1. Broadband pulsed laser; 2. Fiber optic circulator; 3. Fiber optic probe; 4. Vibrating test object; 5. Fiber optic acousto-optic modulator; 6. Fiber optic amplifier; 7. Fiber optic spectrometer. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0047] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0048] Example 1: A method for measuring vibration using an all-fiber optic cable based on frequency domain interferometry, such as... Figure 1 As shown, it includes the following steps:

[0049] S1: Emits a sequence of repeating pulsed lasers with broad spectral characteristics;

[0050] S2: Emits a square wave signal with the same repetition frequency as the pulsed laser, and adjusts the timing of the optical path operation according to the square wave signal;

[0051] S3: Collects the signal light and reference light generated after single pulses in pulsed laser light are used to measure the displacement of the vibrating test object;

[0052] S4: When the optical path is open, the signal light and the reference light are amplified to obtain an optical signal with frequency domain interference characteristics;

[0053] S5: Perform Fourier transform on the frequency domain interference characteristics in the optical signal to obtain the vibration displacement of the vibrating object under test.

[0054] This invention addresses the shortcomings of traditional accelerometer and optical vibration measurement technologies in terms of sensor size, measurement methods, and anti-interference capabilities. Based on the principle of optical frequency domain interferometry, it designs an all-fiber, non-contact optical vibration measurement method. By using a sequence of repeating pulsed lasers with broad spectral characteristics to measure vibration, the sampling frequency is increased. The measurement timing is precisely controlled, dynamic ambiguity is suppressed and amplified, and Fourier transform is used for accurate signal analysis, achieving kHz-level vibration process measurement.

[0055] In this embodiment, the spectral half-width of the pulsed laser is not less than 5nm, the pulse half-width is not greater than 20ns, and the repetition frequency is not greater than 50kHz, which can measure vibration processes with a frequency not exceeding 25kHz. In addition, the upper limit of amplitude measurement can reach 6mm, the accuracy can be better than 0.01mm, and the resolution can reach 2.4μm. Furthermore, within the pulse time, the maximum vibration displacement is only 2.4μm, which is not greater than the measurement resolution, so it will not cause dynamic blurring.

[0056] The signal characteristics formed by the single-pulse signal light and reference light in the fiber optic spectrometer are optical signals with frequency domain interference characteristics, that is, the light intensity changes periodically with frequency. The specific expression of the frequency domain interference characteristics is as follows:

[0057]

[0058] Where I(f,t) represents the optical energy spectral density, f is the optical frequency, and t is the pulse laser emission time; E0(f) represents the optical electric field intensity; and T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the pulse laser emission time. This represents the inherent phase difference between the signal light and the reference light.

[0059] Furthermore, by performing a Fourier transform on the frequency domain interference characteristics, the characteristic time can be obtained. Combined with the characteristic time, the vibration displacement of the vibrating object can be calculated. The specific calculation formula is as follows:

[0060] T(t) = 2d(t) / c

[0061] Where T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the moment of pulse laser emission; d(t) represents the vibration displacement of the surface of the vibrating object due to vibration; and c represents the speed of light in a vacuum.

[0062] Example 2: A full-fiber vibration measurement system based on frequency domain interferometry, such as Figure 2 As shown, the system is used to implement a frequency domain interferometry-based all-fiber vibration measurement method described in Example 1, including a broadband pulsed laser 1, an optical path control circuit, a displacement measurement circuit, a signal processing circuit, and a fiber optic spectrometer 7.

[0063] The system includes: a broadband pulsed laser 1, which emits a series of repeating pulsed lasers with broadband spectral characteristics; an optical path control circuit, built into the broadband pulsed laser 1, which synchronously outputs a square wave signal with the same repetition frequency as the pulsed laser, and controls the opening and closing of the optical path according to the high and low voltage of the square wave signal; a displacement measurement circuit, which collects the signal light and reference light formed after the single pulses in the pulsed laser measure the displacement of the vibrating test object 4; a signal processing circuit, which amplifies the signal light and reference light when the optical path is open, to obtain an optical signal with frequency domain interference characteristics; and a fiber optic spectrometer 7, which performs Fourier transform on the frequency domain interference characteristics in the optical signal to obtain the vibration displacement of the vibrating test object 4.

[0064] Specifically, the displacement measurement circuit includes an optical fiber circulator 2 and an optical fiber probe 3; the three ports of the optical fiber circulator 2 are respectively connected to the output of the broadband pulsed laser 1, the input of the optical fiber probe 3, and the input of the optical fiber acousto-optic modulator 5 in the signal processing circuit. It should be noted that the optical fiber circulator 2 can be replaced by an optical fiber coupler.

[0065] In this embodiment, the fiber end face of the fiber optic probe 3 is either uncoated or coated with a dielectric antireflection film to adjust the return loss, thereby controlling the intensity ratio of the reference light and the signal light.

[0066] In addition, the optical path control circuit is built into the broadband pulsed laser 1, which synchronously outputs a square wave signal with the same repetition frequency as the pulsed laser; the trigger output terminal of the broadband pulsed laser 1 is connected to the drive terminal of the fiber optic acousto-optic modulator 5 in the signal processing circuit, and controls the opening and closing of the fiber optic acousto-optic modulator 5 according to the high voltage and low voltage of the square wave signal; the trigger output terminal of the broadband pulsed laser 1 is also connected to the trigger input terminal of the fiber optic spectrometer 7 to control the fiber optic spectrometer 7 to work synchronously.

[0067] In addition, the signal processing circuit includes an optical fiber acousto-optic modulator 5 and an optical fiber amplifier 6 connected sequentially along the signal output direction.

[0068] In this embodiment, the fiber optic spectrometer 7 uses a grating as a dispersive element, with a spectral width smaller than the spectral range of the output laser of the broadband pulsed laser 1, and a spectral sampling interval no greater than 1 / 4 of the spectral resolution.

[0069] In the optical links described above, the optical fibers of each device can be single-mode optical fibers conforming to the International Telecommunication Union (ITU-T) G.657. The optical fibers are connected by optical fiber connectors or fusion splices. In the electronic links, coaxial cables and matching connectors are used for connection.

[0070] Working principle: A series of repeating pulsed lasers with broadband characteristics emitted by broadband pulsed laser 1 are sent to fiber optic probe 3 through the ①-② ports of fiber optic circulator 2.

[0071] Using the Fresnel reflection principle, a single pulse of light is split into two pulses at the fiber end face of the fiber optic probe 3; one pulse of light returns directly to the fiber optic circulator 2 and becomes the reference light; the other pulse of light illuminates the vibrating test object 4, is reflected by its surface, and is collected by the fiber optic probe 3 and sent back to the fiber optic circulator 2 as the signal light; the reference light and the signal light enter the fiber optic acousto-optic modulator 5 through the ②-③ ports of the fiber optic circulator 2.

[0072] The fiber optic acousto-optic modulator 5 functions as an "optical switch" in the optical path. Its on / off control process is as follows: Simultaneously, the broadband pulsed laser 1 emits a pulsed laser signal, which is then input to the trigger terminal of the fiber optic acousto-optic modulator 5 via the trigger output terminal. This acousto-optic effect changes the direction of the beam entering the modulator 5. Specifically, when a high voltage square wave signal arrives, the optical path opens, allowing the pulsed reference light and signal light to travel from the input terminal to the output terminal of the modulator 5. When a low voltage square wave signal arrives, the optical path closes, preventing the pulsed reference light and signal light from traveling from the input terminal to the output terminal. When the modulator 5 is open, the signal light and reference light are transmitted to the fiber optic amplifier 6. After optical amplification, they are sent to the fiber optic spectrometer 7 for recording and analysis; alternatively, the fiber optic spectrometer 7 can record the signal and transmit it to a host computer for analysis.

[0073] Example 3: Using the technical solutions in Examples 1 and 2, the vibration behavior characteristics of a horn were measured. The only difference in the measurement method was that the vibrating test object 4 was replaced with a planar resonant horn. In the experiment, the vibration frequencies of the horn were 500Hz, 600Hz, 700Hz, 800Hz, 900Hz, 1000Hz, 1200Hz, 1500Hz, and 2000Hz. Some measurement results are shown in Figure 3 and... Figure 4 As shown, the maximum amplitude was measured to be 0.040 mm and the minimum amplitude was 0.012 mm. The modal information of vibration at each frequency was obtained by Fourier transform.

[0074] Furthermore, based on the basic error measurement methods specified in the national metrological standard "Calibration Specification for Linear Displacement Sensors" (JJF1305-2011), a linear translation stage with a positioning accuracy of 2μm was used as the displacement reference to evaluate the measurement accuracy of the above-mentioned technical solution. The results obtained in the evaluation experiment are as follows: Figure 5 As shown, the measurement range is 1.17–5.73 mm, the basic error is 0.009 mm, and the measurement accuracy is high.

[0075] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A full-fiber vibration measurement system based on frequency domain interferometry, characterized in that, include: A broadband pulsed laser (1) is used to emit a series of repeating pulsed lasers with broadband characteristics, wherein the half-width of the spectral laser is not less than 5 nm, the half-width of the pulse is not greater than 20 ns, and the repetition frequency is not greater than 50 kHz. The optical path control circuit is used to emit a square wave signal with the same repetition frequency as the pulsed laser, and to regulate the timing of the optical path operation according to the square wave signal. The displacement measurement circuit is used to collect the signal light and reference light formed after the single pulse in the pulsed laser measures the displacement of the vibrating test object (4); The signal processing circuit is used to amplify the signal light and the reference light when the optical path is open, so as to obtain an optical signal with frequency domain interference characteristics. Fiber optic spectrometer (7) is used to perform Fourier transform on the frequency domain interference characteristics in the optical signal to obtain the vibration displacement of the vibrating test object (4); The displacement measurement circuit includes an optical fiber circulator (2) and an optical fiber probe (3); The three ports of the fiber optic circulator (2) are respectively connected to the output end of the broadband pulsed laser (1), the input end of the fiber optic probe (3), and the input end of the fiber optic acousto-optic modulator (5) in the signal processing circuit. The fiber end face of the fiber probe (3) is either uncoated or coated with a dielectric antireflection film to adjust the return loss, thereby controlling the intensity ratio of the reference light and the signal light. The optical path control circuit is built into the broadband pulsed laser and synchronously outputs a square wave signal with the same repetition frequency as the pulsed laser. The trigger output terminal of the broadband pulsed laser (1) is connected to the trigger terminal of the fiber acousto-optic modulator (5) in the signal processing circuit, and the fiber acousto-optic modulator (5) is turned on and off according to the high voltage and low voltage of the square wave signal. The trigger output terminal of the broadband pulsed laser (1) is connected to the trigger input terminal of the fiber optic spectrometer (7) to control the fiber optic spectrometer (7) to work synchronously. The signal processing circuit includes an optical fiber acousto-optic modulator (5) and an optical fiber amplifier (6) connected sequentially along the signal output direction; The fiber optic acousto-optic modulator (5) is used as an optical switch in the optical path, and its switching action control process is as follows: When the square wave signal high voltage arrives, the optical path is opened, and the pulse reference light and signal light are transmitted from the input end of the fiber optic acousto-optic modulator (5) to the output end. When the square wave signal low voltage arrives, the optical path is closed, and the pulse reference light and signal light cannot be transmitted from the input end of the fiber optic acousto-optic modulator (5) to the output end. When the fiber optic acousto-optic modulator (5) is opened, the signal light and reference light are transmitted to the fiber optic amplifier (6), and after optical amplification, they are sent to the fiber optic spectrometer (7) for recording and analysis.

2. A method for measuring vibration using an all-fiber optic cable based on frequency domain interferometry, characterized in that, The all-fiber vibration measurement system based on frequency domain interferometry as described in claim 1 includes the following steps: It emits a series of repeating pulsed lasers with broad spectral characteristics; It emits a square wave signal with the same repetition frequency as the pulsed laser, and adjusts the timing of the optical path operation according to the square wave signal; The signal light and reference light generated after single pulses from pulsed lasers are used to measure the displacement of a vibrating object under test are collected. When the optical path is open, the signal light and the reference light are amplified to obtain an optical signal with frequency domain interference characteristics. The vibration displacement of the vibrating object is obtained by performing a Fourier transform on the frequency domain interference characteristics of the optical signal. The pulsed laser has a spectral half-width of not less than 5 nm, a pulse half-width of not more than 20 ns, and a repetition frequency of not more than 50 kHz.

3. The all-fiber vibration measurement method based on frequency domain interferometry according to claim 2, characterized in that, The frequency domain interference characteristic expression of the optical signal is specifically as follows: ； Where I(f,t) represents the optical energy spectral density, f is the optical frequency, and t is the pulse laser emission time; T(t) represents the electric field intensity of the light wave; T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the moment of pulse laser emission. This represents the inherent phase difference between the signal light and the reference light.

4. The all-fiber vibration measurement method based on frequency domain interferometry according to claim 2, characterized in that, The specific formula for calculating the vibration displacement of the vibrating object under test is as follows: T(t) = 2d(t) / c; Where T(t) represents the characteristic time of the periodic change of the frequency domain interference signal at the moment of pulse laser emission; d(t) represents the vibration displacement of the surface of the vibrating object due to vibration; and c represents the speed of light in a vacuum.

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